Elements of natural philosophy (Volume 2-3) . 19S NATURAL PHILOSOPHY. Number ofdegrees in samearc referred tothe centre F; in which * denotes the ratio of the diameter of a circleto its circumference. When this arc a is referred to the centre F\ corres-ponding to a radius f, its number of degrees, denotedby », becomes, , a . 360° 1n = —-, and dividing the first of these Equations by the second,we find, Eatio of theabove values; n 1 7T Conclusion for diverging rays. whence we conclude that — and — measure the relative f f divergence of the incident and deviated rays. When the devi-ated rays mee
Elements of natural philosophy (Volume 2-3) . 19S NATURAL PHILOSOPHY. Number ofdegrees in samearc referred tothe centre F; in which * denotes the ratio of the diameter of a circleto its circumference. When this arc a is referred to the centre F\ corres-ponding to a radius f, its number of degrees, denotedby », becomes, , a . 360° 1n = —-, and dividing the first of these Equations by the second,we find, Eatio of theabove values; n 1 7T Conclusion for diverging rays. whence we conclude that — and — measure the relative f f divergence of the incident and deviated rays. When the devi-ated rays meetthe axis at F\ onthe opposite sideof the deviatingsurface from theradiant, the value/, being laid offin a contrary di-rection from the. Conclusion for converging rays; vertex 0, becomes negative, and the relative measure / / 5 for the convergence of these rays will be , if the incident rays converge to a point F,before deviation, f for the same reason, wrould be ne-gative, and the measure for the corresponding conver-gence would be negative. And, generally, we shall findthat, referring the radiant and focal distances to the ELEMENTS OF OPTICS. 199 vertex as an origin, di-vergence will be mea-sured by a positive and ^\ ^^convergence by a nega- ^ry\tive quantity; and for <-<// \ . J ^ / V \ General rule for Convenience We Shall, <^^j £ \ vergency of rays. therefore, hereafter em- ^\v0\ / ploy the general term ^^L/ vergency to express either /^Nv of these conditions of therays, indifferently. § 39. The power of a lens is its greater or less capacity Power of a deviate the rays that pass through it. In Equations (33), (31), (35,) &c, * * ^—, &c, ^~ a mi nun will measure the verge
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